Gas diffuser with indefinite life and mixed bubble size capacity

ABSTRACT

A diffuser for dispersing gas into a body of liquid that may contain suspended solids. Under ordinary clogging conditions, the diffuser will remain operative over an indefinite period of time by reason of large gas outlet openings at the bottom of the gas plenum chamber. In addition, it can be employed for a good portion of that time to disperse a substantial flow of coarse bubbles and fine bubbles, in a desired combination, simultaneously from a single gas plenum. Careful selection of the permeability and area of a top porous wall of a gas plenum chamber, together with proper selection of the level, size and number of coarse bubble gas outlet openings in the side walls of the plenum chamber, achieve the second feature.

This invention relates to a diffuser for dispersing gas into a body ofliquid that may contain suspended solids, and in particular such adiffuser that under ordinary clogging conditions will remain operativeover an indefinite period of time, and in addition can be employed for agood portion of that time to disperse a substantial flow of both coarsebubbles and fine bubbles, in a desired combination, simultaneously froma single gas plenum.

BACKGROUND OF THE INVENTION

During use of any gas diffuser that is operated while submerged in abody of liquid that may contain suspended solids, such as waste water,eventual clogging of all outlet openings below a certain size in thewalls of the gas plenum chamber of the diffuser is unavoidable becauseof dirt in the gas (such as air) that is pumped through the diffuser,the settling out of solid particles of various types that are suspendedin the liquid, and the formation of deposits of iron or calciumcompounds or other similar compounds. If such a diffuser is used underordinary conditions of use for a long enough period of time, experiencehas shown that in some liquids any gas outlet openings that produce gasbubbles that are approximately 1/2" or smaller in diameter are likely tobecome completely clogged.

Gas diffusers are known that address this problem by providing, inaddition to smaller openings, some alternative gas outlet openings thatare large enough that they will never become clogged under any likelyconditions of use. Thus, gas diffusers for operating in a body of liquidare known that include two groups of gas outlet openings from the gasplenum chamber, the first group being of smaller size and located at ahigher level than the second group, with the openings in the secondgroup being horizontal slots large enough that even if the openings inthe first group become completely clogged after a long period of use,the larger slots will remain open indefinitely (under any ordinaryconditions of use) no matter how long the diffuser remains submerged inthe body of liquid into which gas is being introduced. The gas diffuserdisclosed in Thayer U.S. Pat. No. 3,424,443 issued Jan. 28, 1969 is oneexample of such a device.

In the device of the Thayer patent, gas is introduced at the top of theplenum chamber to displace liquid that initially flowed into the plenumfrom the body of liquid in which the plenum chamber is submerged. Afterenough gas has accumulated under the impermeable top wall andimpermeable upper wall portions of the plenum chamber to lower thesurface of the liquid in the chamber to the level of the smaller outletopenings in the side walls of the chamber, the gas flows out throughthese smaller openings. So long as these smaller openings do not closeup completely from the clogging that (as explained above) unavoidablyoccurs during extended periods of diffuser use, some gas is permitted toexit through the side walls.

The resulting bubbles grow smaller as the openings gradually become moreand more clogged. In addition, the Thayer device attempts to decreasethe size of the bubbles from the side wall openings by constructing thebottom wall of the gas plenum chamber in a special configuration that isintended to give the upwardly directed liquid currents (resulting fromthe rising gas bubbles after they are emitted from the side wallopenings) a shearing effect as they move upward past the side walls ofthe chamber.

After the side wall openings of the gas plenum chamber in a device suchas the Thayer diffuser have become completely clogged, the pressurizedgas enclosed within the plenum beneath the impermeable top wall--and nowthe impermeable side walls--pushes the surface of the liquid in thelower portions of the plenum chamber down to the level of the horizontal"emergency air release slots" at the bottom of the chamber (col. 2,lines 25-26).

The Thayer invention thus achieves a gas diffuser having an indefinitelife through utilization of a gas plenum chamber having an impermeabletop and impermeable upper wall portions to produce gas flow on aconsecutive basis from the same gas plenum--initially through first,smaller gas outlet openings at a level part way down the side walls ofthe plenum chamber and after those openings become completely clogged,through second, much larger gas outlet openings in the form ofhorizontal slots at the bottom of the plenum chamber (col. 1, line 70 tocol. 2, line 2; col. 3, lines 43-47).

U.S. Pat. No. 3,608,834 to MacLaren discloses an air diffuser similar tothe Thayer device in that it utilizes openings in the side walls of theplenum chamber and notches at the open bottom of the chamber. The plenumchamber of the MacLaren diffuser, like that of the Thayer diffuser, hasnot only impermeable upper wall portions but also an impermeable topwall (except for an inlet valve that opens to admit air into the plenumchamber during operation of the device), and thus the two diffusersoperate basically in much the same way.

In the Thayer and MacLaren difusers, although both air ports 23 andopenings 57 spaced above the bottoms of the respective side walls of thediffusers are smaller than longitudinal air escape slots 27 and notches46 or 56 at the bottom of the side walls, they are still not smallenough to produce fine gas bubbles as defined below in thisspecification. Furthermore, even if they were small enough, the bubblesemitted from Thayer ports 23 and MacLaren upper openings 57 are formedonly in single narrow zones along the perimeters of the respective gasdiffusers, which zones are defined by the side walls of the plenumchambers, instead of across the entire porous area of the top wall ofthe diffuser, as is the case with the present invention.

SUMMARY OF THE INVENTION Simultaneous Production From Single Gas PlenumOf Desired Mix Of Fine And Coarse Bubbles

Applicant has unexpectedly discovered that it is not necessary to makethe top wall of the gas plenum impermeable as Thayer and MacLaren dobut, on the contrary, there are definite advantages to making the topwall permeable if its permeability is carefully selected and certainadditional gas outlet openings of a larger size are employed at acarefully selected level part way down the side walls of the gas plenumchamber. Specifically, if (1) the permeability of the top wall of thegas plenum chamber is properly selected to produce fine gas bubbles (asdefined below in this specification), and (2) small gas outlet openingsare positioned in the plenum chamber side walls at the proper levelbelow the porous top wall of the chamber, it is possible during asubstantial portion of the life of the diffuser to achieve simultaneousproduction of a desired "mix" or proportions of fine bubbles and coarsebubbles (as defined below in this specification) from a single gasplenum.

Applicant has further discovered that if the two parameters mentionedabove (permeability of top wall and level of gas outlet openings in sidewalls) are properly selected, the particular ratio of the quantity offine bubbles to the quantity of coarse bubbles that is appropriate forthe application for which the diffuser is to be used can be achievedduring the first period of diffuser use, and thereafter roughlymaintained so long as the porous material and other gas outlet openingsfrom the gas plenum chamber do not become clogged at rates that are toosharply different from each other.

Desired Proportions Of Fine And Coarse Bubbles

As is well known, fine gas bubbles achieve a higher gas transfer rate inthe liquid into which they are introduced than can be achieved withlarger bubbles. On the other hand, coarse gas bubbles have the advantageof achieving a greater mixing effect by helping to keep any solidparticles that are suspended in the body of liquid in suspension.

Many diffusers are known that disperse fine gas bubbles into a body ofliquid in which they are immersed. Similarly, many diffusers are knownfor the production and dispersion of coarse gas bubbles into a body ofliquid in which those diffusers are immersed. So far as is known toapplicant, however, before the present invention there has never been adiffuser device that simultaneously produces and disperses from a singlegas plenum gas bubbles both fine and coarse in size as defined in thisspecification.

Since applicant has discovered unexpectedly that his invention makes itpossible to produce a substantial quantity of fine bubbles and asubstantial quantity of coarse bubbles simultaneously and in desiredproportions from a single gas plenum, this invention makes available atone and the same time the above described advantages of both fine bubblediffusion and coarse bubble diffusion, with a substantial savings inequipment costs by reason of the fact that only a single plenum isrequired for both functions.

Because of the respective advantages of a flow of fine gas bubbles and aflow of coarse gas bubbles, and the accompanying disadvantage with finebubbles of a low gas flow rate and resulting low gas transferefficiency, a different "mix" or different proportions of fine andcoarse bubbles sizes is preferred for various uses to which a gasdiffuser is put. In standard waste water treatment such as in a sewagetreatment plant, for example, a good balance is believed to beapproximately 80 percent by flow rate of fine air bubbles andapproximately 20 percent by flow rate of coarse air bubbles. In anaeration channel, in which a greater mixing action is required, it isbelieved that a good balance is approximately 50 percent by flow rate offine air bubbles and approximately 50 percent by flow rate of coarse airbubbles. For other applications, the mixture of fine and coarse bubblesthat is desired will be determined by the respective requirements (knownto those skilled in the art) as to gas transfer efficiency and mixingthat are presented by the applications in question.

No gas diffusers prior to the device of the present invention that areknown to applicant could achieve not only simultaneous production from asingle gas plenum of a substantial quantity of fine gas bubbles and asubstantial quantity of coarse gas bubbles, but in desired predeterminedproportions as well.

Definitions

As used in this specification and claims, the indicated terms have thefollowing meanings:

1. The term "fine gas bubbles" refers to bubbles that have a diameter ofno more than approximately 5 mm. (just under 1/4"), which is the meaninggenerally given the term in the sewage treatment field. There is as apractical matter no precise line of distinction between fine gas bubblesand coarse gas bubbles, but a rough line of demarcation can be drawn. Inthis specification and claims, the term "coarse gas bubbles" is used torefer to bubbles that have a diameter of approximately 10 mm. (just over3/8") and larger.

2. Any measurement as to "standard cubic feet per minute" is understoodto be made at 60° F., 14.696 p.s.i.a., and 760 mm. Hg.

3. The term "stabilized wet permeability" of a given porous materialmeans the amount of air in standard cubic feet per minute that will flowat 70° F. and 25 percent relative humidity through one square foot of a1" thick piece of the material under 2 inches of water differentialpressure, with the air discharging into 48" of water above the porousmaterial. If the porous material is saturated with water at thebeginning of the test, then air flow shall continue through the materialuntil all water held by capillary attraction within the porous materialis expelled and stabilized flow is established.

As will be seen, the units for the "stabilized wet permeability" of anygiven porous material are:

Standard cubic feet per minute (scfm)/70° F./25 percent rel. hum./ft.²/1" thick/2" H₂ 0 pressure/into 48 inches of H₂ 0.

To avoid undue complications, whenever this parameter is referred to inthe present specification and claims, its units will be omitted, and thevalue for the stabilized wet permeability of a given porous materialwill be expressed simply as a numerical figure, with the unitsunderstood.

The definition of "standard cubic feet per minute" given above is theaccepted standard definition of that term. The definition of "stabilizedwet permeability" is consistent with the accepted standard definition of"stabilized dry permeability," revised to reflect the fact that when theparameter of wet permeability is being determined, the air beingmeasured flows through the porous material into water.

Summary Of Structure Of Diffuser Of This Invention

In its broadest form, the gas diffuser of this invention comprises achamber that forms a gas plenum and includes:

1. A top wall at least a substantial portion of which is formed of aporous material having a stabilized wet permeability of a magnitude tocause the production of fine gas bubbles;

2. Side walls defining, at a predetermined level below said top wall, aplurality of gas outlet openings of a first predetermined size toproduce coarse gas bubbles; and

3. A gas inlet opening, preferably at a level below the porous top walland above the plurality of coarse bubble outlet openings.

When gas is introduced into the gas plenum chamber through the gas inletopening, it will fill at least the upper portion of the plenum and fromthere exit through the porous top wall to form fine gas bubbles in thebody of liquid in which the diffuser is submerged. When gas isintroduced at a higher pressure, it will be discharged from the plenumsimultaneously (1) through the porous top wall to form fine gas bubbles,and (2) through the plurality of outlet openings below the level of thegas inlet opening to produce coarse gas bubbles.

One embodiment of the diffuser of this invention includes a bottom wallspaced below the side walls of the gas plenum chamber to form aplurality of horizontal slots large enough to avoid clogging during anindefinite period under ordinary conditions of use. In this embodiment,if the permeable top wall and the side wall gas outlet openings becomeclogged after an extended period of diffuser use, the horizontal slotsat the bottom of the plenum chamber will remain open for an indefinitetime under any ordinary conditions of use.

Control Of Two Factors Achieves Desired Mix Of Fine And Coarse Bubbles

It will be seen that applicant has made possible the production ofvarious mixes of fine gas bubbles and coarse gas bubbles from the samegas plenum by controlling two parameters that affect the quantities ofeach size of bubbles produced by the diffuser of this invention:

(1) The pore size or permeability of the top wall of the gas plenumchamber; and

(2) The differential pressure to which the top wall of the gas plenumchamber, through which gas flows out of the gas plenum, is subjected.

The first of these two parameters is determined by the selection of thepore size, outlet opening size, or stabilized wet permeability (asdefined above) of the porous material in the top wall of the gas plenumchamber. The second is determined--since the water level in the gasplenum chamber must drop to the level of the gas outlet openings forcoarse gas bubbles before any of the pressurized gas can flow outthrough those coarse bubble openings--by the distance below the poroustop wall of the plenum chamber that those gas outlet openings arelocated.

Other parameters in the construction of the gas diffuser of thisinvention that affect the proportions of fine and coarse bubbles ofwhich the diffuser is capable include (1) the area of the top wall ofthe gas plenum chamber that is comprised of porous material, (2) thesize of the side wall gas outlet openings, and (3) the number of suchopenings. In addition, the mix of fine and coarse bubbles produced bythe diffuser is affected, as will be discussed below, by the interactionbetween the level of the side wall gas outlet openings and the pressureat which the gas is introduced into the gas plenum chamber duringoperation of the diffuser.

Permeability Of Porous Top Wall

Satisfactory results are obtained with the gas diffuser of thisinvention that includes a top wall for the gas plenum chamber formed ofa porous material having a stabilized wet permeability of not more thanabout 5. Improved results are obtained with a stabilized wetpermeability of more than about 10 but less than about 35. Still furtherimproved results are obtained with a stabilized wet permeability rangeof about 5 to about 10, with the preferred value being about 7.5. Asalready explained above, to determine these parameters the resultsachieved by use of the diffuser of this invention are evaluated in termsof the level of production of fine gas bubbles and the level of thesimultaneous production of some larger bubbles.

BRIEF DESCRIPTION OF THE DRAWING

This invention will be described by reference to the accompanyingdrawing, in which:

FIG. 1 is a plan view of a gas diffuser comprising one embodiment of theapparatus of this invention, with a portion of a gas supply pipe shownconnected therewith;

FIG. 2 is a sectional view of the embodiment of FIG. 1, taken along theline 2--2 in that Figure; and

FIG. 3 is a fragmentary sectional view of another embodiment of theapparatus of this invention.

DETAILED DESCRIPTION OF TWO EMBODIMENTS OF THIS INVENTION The StructureOf The Diffuser

FIG. 1 is a plan view of gas diffuser 10 of this invention submerged ina body of transparent liquid 12. Diffuser body or chamber 14 has acentral inlet opening 16 at its lower end and a larger outlet opening 18at its upper end. Porous member 20, which is constructed so that whengas diffuser 10 is submerged in body of receiving liquid 12 the uppersurface of member 20 will be horizontal throughout its area, closesoutlet opening 18 with an airtight fit.

Chamber 14 forms gas plenum 22. The chamber includes porous top wall 20,side walls 24, and bottom wall 26 (the latter best seen in FIG. 2).

As seen in FIG. 2 (which is a cross sectional view taken along line 2--2in FIG. 1), except for sealant 28, all of top wall 20 is formed of aporous material having a stabilized wet permeability of a magnitude tocause the production of fine gas bubbles in body of liquid 12 when gasunder pressure is introduced into plenum 22 through central inletopening 16.

As seen from all three Figures of the drawing, both because of thethickness of side walls 24 and because of sealant 28 (when the latter ispresent), the porous portion of top wall 20 is spaced inwardlythroughout its perimeter from the exterior surface of the side walls ofthe plenum chamber.

Gas inlet stem 30 is positioned within chamber 14 in fluid communicationwith central opening 16. The upper end 36 of stem 30 forms the gas inletopening into gas plenum 22.

Side walls 24 define, at a predetermined distance d₁ below top wall 20,a plurality of gas outlet openings 32. In the embodiment shown, gasplenum chamber 14 has an over-all diameter of approximately 6", and gasoutlet openings 32 have a diameter of approximately 1/8".

The size of gas outlet openings 32 is selected to produce coarse gasbubbles when gas is introduced into gas plenum 22 at a pressure that isgreat enough to force the level of liquid within the gas plenum down adistance greater than distance d₁ below top wall 20. This level isindicated in phantom in FIG. 2. With the liquid within gas plenum 22 atthis level, gas can exit through openings 32 and be discharged into bodyof liquid 12.

As will be seen from FIGS. 1 and 2, outlet openings 32 are of apredetermined, fixed size, and are substantially equally spaced aroundthe perimeter of the chamber. They are unobstructed at all times exceptfor any deposit that may be built up in the openings during use of thediffuser.

In the embodiment shown, side walls 24 of gas plenum chamber 14 define asecond plurality of gas outlet openings 34 that are larger than gasoutlet openings 32. Openings 34 are positioned at distance d₂ below topwall 20, which is at a level below the level of outlet opening 32.

The gas diffuser of this invention can be used to advantage without thesecond level of gas outlet openings 34, but with those openings someflexibility in coarse bubble size is provided, and the length of timethe gas diffuser can be used before the gas outlet openings in sidewalls 24 become clogged is extended because of the larger size ofopenings 34. In the embodiment shown, each opening 34 has a diameter ofapproximately 3/8".

As will be seen from FIG. 2, in this embodiment gas inlet opening 36 atthe upper end of stem 30 is located at a level below porous top wall 20and above coarse bubble outlet openings 32 and 34.

Gas supply pipe 40 is positioned below gas diffuser 10. Gas plenumchamber 14 is supported on supply pipe 40 by diffuser saddle 42.

The supply pipe has an inlet end 44 for introduction of gas to be pipedto and into gas plenum 22. The remote end of supply pipe 40 is connectedto a source (not shown) of the gas that is to be diffused into the bodyof liquid being treated.

Combination diffuser inlet tube and supply pipe drainer stem 46 extendsgenerally vertically downward from central inlet opening 16 into supplypipe 40 (FIG. 2). Its lower inlet end 48 extends to a location adjacentbut spaced from bottom wall 50 of supply pipe 40. Member 46 has a dualfunction--introduction of gas into diffuser 10 and discharge of unwantedliquid from supply pipe 40 when the diffuser is started up after aninactive period during which liquid has entered plenum 22 and theinterior of supply pipe 40. The operation of member 46 is explained incommonly assigned co-pending application Ser. No. 638,776 entitled "GasDiffuser And Accompanying Piping System."

In the embodiment shown, bottom wall 26 of gas plenum chamber 14 isspaced below side walls 24 to form a plurality of horizontal slots 52.Slots 52 provide outlet openings between bottom wall 26 and side walls24 that are large enough to avoid clogging during an indefinite periodunder ordinary conditions of use. As seen, four spacer members 54 andassociated bolts 56 support bottom wall 26 on side walls 24 to form theslots.

In this embodiment, slots 52 are approximately 1/2" wide andapproximately 4" long. As a result, the slots extend throughout asubstantial portion of the perimeter of side walls 24. As in the case ofgas outlet openings 32 and 34, the approximate level below which theliquid within gas plenum chamber 14 is pushed down by pressurized gaswithin plenum 22 to permit gas to exit from the plenum through slots 52is shown at distance d₃ below top wall 20.

FIG. 3 shows porous top wall 60 of gas plenum chamber 14. As with theembodiment of FIGS. 1 and 2, the porous top wall is connected throughsealant 62 with side walls 14 to form an airtight seal.

In this embodiment, top wall 60 is formed of a tightly woven wire screenmaterial.

Operation Of Gas Diffuser Of This Invention

To sum up, the use of the gas diffuser of this invention makes possiblethe simultaneous production of desired proportions of both fine andcoarse gas bubbles from a single gas plenum, and in one form permitsoperation of the diffuser to provide large, irregularly shapedquantities of gas (which in the vernacular might be referred to as("blurps" of gas) for an indefinite period of time after all smaller gasoutlet openings have become completely clogged.

As has been pointed out, the degree of permeability of the porous topwall of the gas plenum chamber determines the size of the fine gasbubbles that are produced by the diffuser and the size of the gas outletopenings in the chamber side walls determines the size of the coarse gasbubbles produced--until the porous material of the top wall and the sidewall outlet openings become clogged through extended diffuser use.

The proportions of fine gas bubbles and coarse gas bubbles that areproduced are determined by the parameters discussed above--the degree ofpermeability and area of the porous top wall, and the vertical level,size and number of the coarse bubble openings in the side walls--asaffected by the gas pressure that is maintained in the gas plenumchamber.

The horizontal slots at the bottom of the gas plenum chamber of thediffuser of this invention are selected to be sufficiently large thatthere will never be clogging of these slots under any ordinaryconditions of use.

As mentioned above, satisfactory results are obtained with the gasdiffuser of this invention as described above and shown in the drawingusing a porous material having a stabilized wet permeability of lessthan about 5. With porous materials falling in this range, a smallvolume of air will flow through the porous material at the top of thegas plenum chamber to create fine bubbles of 3 mm. or less in diameter,and a large volume of air will flow through the 1/8" diameter and 3/8"diameter holes on the side walls of the diffuser. The large volume ofair exiting through the 1/8" and 3/8" diameter holes will create largeirregularly shaped bubbles having a minimum dimension of approximately3/8" to 1", depending on header supply pressure. In the event that theporous material and the holes become plugged completely, air willbegin--and continue indefinitely--to exit in extremely large,irregularly shaped quantities of air (which as suggested above might bereferred to as "blurps" of air) through the horizontal slots at thebottom of the gas plenum chamber of the diffuser.

Improved results are obtained with a porous material in the top wall ofthe gas plenum chamber having a stabilized wet permeability of more thanabout 10 and less than about 35. Within this range of permeabilities,the air will initially flow through the top porous material and willproduce medium size bubbles in the range of 3 to 10 mm. In the eventthat the pressure at which gas is introduced into the gas plenum chamberis increased sufficiently, or in the event that enough plugging orfouling of the porous material occurs, the pressure drop across theporous material will increase to such a point that air flow willcommence through the side wall holes as well as through the top porousmaterial. Complete plugging of the top porous material and the side wallholes will cause air flow to begin--and continue indefinitely--throughthe slots at the bottom of the plenum chamber of the diffuser.

Still further improved results are obtained with a stabilized wetpermeability range of about 5 to about 10, with the preferred valuebeing about 7.5. At a stabilized wet permeability of 7.5, the diffusercan easily be controlled to produce simultaneously approximately 3 mm.nominal diameter bubbles from the porous material and 3/8" to 1" minimumdimension bubbles from the holes in the diffuser side walls. With d₁equal to 41/2" and d₂ equal to 5", controlling the diffuser plenumpressure to less than 41/2" of water causes air to flow through the topporous material only. Plenum pressures of 41/2" to 5" in water causesair to flow through the top porous material and the 1/8" diameter sidewall holes. Plenum pressure greater than 5" of water causes air to flowthrough the top porous material, and through the 1/8" diameter and the3/8" diameter side wall holes as well. In the event that the porousmaterial and the side wall holes are plugged completely or the plenumpressure is increased to 53/4", air will begin--and continueindefinitely--to flow through the horizontal slots at the bottom of theplenum chamber of the diffuser.

EXAMPLE

The following types of air bubble flow--described in terms of flowlocation, flow rate in scfh, and bubble size--are obtained at theindicated operating pressures using a gas diffuser of the type shown anddescribed in this specification, with (1) a circular top wallapproximately 6" in diameter having a stabilized wet permeability of 7.5and side walls having (2) an upper row of eight equally spaced 1/8"holes located approximately 41/2" below the top wall, (3) a lower row ofequally spaced 3/8" holes, located approximately 5" below the top wall,and (4) four horizontal slots 1/2" wide and approximately 4" long at thebottom of the gas plenum chamber:

    ______________________________________                                        Air Plenum           Diffuser Flow                                                                             Bubble                                       Pressure In          In Standard Size -                                       Inches Of                                                                             Flow         Cubic       Minimum                                      Water   Location     Feet/Hour   Dimension                                    ______________________________________                                        1"      No flow       0          0                                            2"      Porous         2.4       approx. 3 mm.                                        material                                                              3"      Porous       20          3 to 4 mm.                                           material                                                              4"      Porous       70          3 to 5 mm.                                           material                                                               41/2"  Porous material                                                                            165*        3 to 5 mm.                                           1/8" diameter            3/8" to 1/2"                                         holes                                                                 5"      Porous material                                                                            350*        3 to 5 mm.                                           1/8" diameter holes      3/8" to 1/2"                                         3/8" diameter holes      1/2" to 1"                                   ______________________________________                                         *Combined flow                                                           

The above detailed description has been given for clarity ofunderstanding only. No unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

I claim:
 1. A gas diffuser for diffusing gas into a body of receivingliquid in which the diffuser is submerged, which comprises a chamberthat forms a gas plenum, said chamber including:(a) a top wall at leasta substantial portion of which is formed of a porous material having astabilized wet permeability of a magnitude to cause the production offine gas bubbles when gas is introduced under a first predeterminedpressure into said gas plenum chamber and exits through said porous topwall; (b) side walls defining, at a predetermined level below said topwall, a plurality of gas outlet openings of a first predetermined, fixedsize to produce coarse gas bubbles when gas is introduced under asecond, higher predetermined pressure into said gas plenum chamber andexits through said gas outlet openings, while continuing at all times toexit simultaneously through said porous top wall and through said gasoutlet openings in said side walls, said gas outlet openings beingunobstructed at all times except for any deposit that may be built up insaid openings during use of said diffuser, said unobstructed gas outletopenings in the side walls of said plenum chamber being substantiallyequally spaced around the perimeter of the chamber; and (c) a gas inletopening,whereby gas introduced under said first predetermined pressureinto said gas inlet opening: will fill the upper portion of said gasplenum and exit through said porous top wall of the plenum chamber toform a rising central cylindrical column of densely packed fine gasbubbles in the body of receiving liquid in which the diffuser issubmerged, and if the pressure at which the gas is maintained in saidplenum is increased to said second predetermined figure, will fill saidplenum chamber down to said predetermined level of said plurality ofcoarse bubble outlet openings and exit through said openings to form arising outer, hollow, cylindrical sheath of coarse gas bubbles in thebody of receiving liquid in which the diffuser is submerged, while saidcolumn of fine gas bubbles continues to be discharged through saidporous top wall of the plenum chamber.
 2. The gas diffuser of claim 1 inwhich said plenum chamber has a central opening adjacent the bottom ofthe chamber, and a gas inlet stem positioned within said chamber is influid communication with said central opening, with the upper end ofsaid stem forming said gas inlet opening into said gas plenum.
 3. Thegas diffuser of claim 1 in which said gas plenum chamber includes abottom wall spaced below said side walls of the chamber to form aplurality of horizontal slots that provide outlet openings between saidbottom wall and said side walls large enough to avoid clogging during anindefinite period under ordinary conditions of use, said slots extendingthroughout a substantial portion of the perimeter of said sidewalls,whereby, if the porous top wall and all said gas outlet openingsin said side walls become completely clogged, gas introduced into saidplenum chamber at any pressure will fill the entire plenum and exitthrough said horizontal slots.
 4. The gas diffuser of claim 1 whichproduces fine gas bubbles and coarse gas bubbles in predeterminedproportions by gas flow rate, in which:(a) the portion of said top wallof the plenum chamber that is formed of said porous material has astabilized wet permeability and a surface area of a magnitude to producea first predetermined flow rate of fine gas bubbles when gas isintroduced into said plenum chamber at the aforesaid first predeterminedpressure, and a second predetermined flow rate of fine gas bubbles whengas is introduced at said second predetermined pressure; (b) said gasoutlet openings of a first predetermined size in the plenum chamber sidewalls are provided in a number to produce a predetermined flow rate ofcoarse gas bubbles when gas is introduced into said plenum chamber atsaid second predetermined pressure,said second flow rate of fine gasbubbles and said flow rate of coarse gas bubbles having saidpredetermined proportions; and (c) said plurality of openings of a firstpredetermined size is located at a level that permits the quantity ofpressurized gas in said plenum to increase, as the pressure at which thegas is introduced into the plenum is increased from said first to saidsecond predetermined pressure, until it causes a pressure drop acrosssaid porous top wall of a magnitude to produce said predeterminedproportion by flow rate of fine gas bubbles, while the pressure of thegas in said plenum chamber is maintained at the aforesaid secondpredetermined pressure to produce ssaid predetermined proportion by flowrate of coarse bubbles.
 5. The gas diffuser of claim 1 in which saidporous material of said top wall of said gas plenum chamber has astabilized wet permeability of no more than about
 35. 6. The gasdiffuser of claim 1 in which said porous material of said top wall ofsaid gas plenum chamber has a stabilized wet permeability of no morethan about
 5. 7. The gas diffuser of claim 1 in which said porousmaterial of said top wall of said gas plenum chamber has a stabilizedwet permeability of more than about 10 but less than about
 35. 8. Thegas diffuser of claim 1 in which said porous material of said top wallof said gas plenum chamber has a stabilized wet permeability of fromabout 5 to about
 10. 9. The gas diffuser of claim 1 in which said porousmaterial of said top wall of said gas plenum chamber has a stabilizedwet permeability of about 7.5.
 10. The gas diffuser of claim 1 in whichsaid gas inlet opening is located at a level below said porous top walland above said plurality of coarse bubble outlet openings.
 11. The gasdiffuser of claim 1 in which the upper surface of said porous portion ofsaid plenum chamber top wall is constructed so as to be horizontalthroughout its area when said diffuser is submerged in said body ofreceiving liquid.
 12. The gas diffuser of claim 1 in which the porousportion of said plenum chamber top wall is inwardly spaced throughoutits perimeter from the exterior surface of said side walls of the plenumchamber.